10,000 years ago, a massive battle known as the Battle Royal was fought by 52 Undead, each representing a species fighting for dominance over all others. The winner was the Human Undead, giving dominion of the Earth to humanity. In the present day, archaeologists discover the sealed Undead, and accidentally set them free. Thus, a new Battle Royal begins.

Having developed the Rider System, based on the Joker's ability to copy sealed Undead, the organization BOARD equips two young men: Kazuma Kenzaki and Sakuya Tachibana fight together as Kamen Riders Blade & Garren to protect humans from Undead and seal them. Also fighting Undead is the mysterious Chalice, a Rider whose purpose is unknown. In addition, a young man struggles to free himself from the control of the sealed Undead powering his own Undead-made Rider System, fighting as Kamen Rider Leangle.